The present invention relates to the general subject of electron optical systems and more particularly to such systems wherein a beam of corpuscular particles is scanned over a selected area of a specimen and the electrical signal derived therefrom is displayed upon a television type monitor (raster driven cathode ray tube).
With the development of electron microprobes systems having a beam of sufficiently high brightness (content of energy) so that high speed scanning of a specimen surface, e.g. scanning at rates comparable to those exhibited by commercial television systems, "Real Time" Scanning Electron Microscopes having high magnification viewable on TV screens have become a commercial reality. With the advent of such "TV-linked" systems have come a number of changes in the style and practice of electron microscopy. Now, an electron beam, having a high concentration of electrons, can be finely focussed (to a spot diameter of a few Angstroms), and caused to scan a specimen surface in synchronism with the electron beam of a CRT monitor.
The impact of the electron beam upon the specimen within the microscope can be detected by any of a variety of devices (x-ray detectors, backscattered electron detectors, etc.) and the signal of such device used to modulate the brightness of the TV tube beam. In the case of the very high brightness sources for electron beams (field emission tips) sufficient signal to noise level can be maintained such that even at very high magnification (50,000.times. or more) a real time, TV rate scan and display may be enjoyed. Scientists and researchers have have found tremendous advantage in being able to immediately view on a TV screen a highly magnified specimen surface, to compare and select the areas of relative interest, and the determination of suitable sights for photomicrographs. As the interest in highly magnified specimen examination forces higher resolution of photomicrographs, advantages of the developments within the TV industry have become available as building blocks for further advance in state of the art of Field Emission microscopy. For example, as TV monitors having higher resolution (higher numbers of lines and a variety of times of persistence phosphors have become available, the scan of the electron beam on the specimen has still been synchronizeable with the TV type monitor, with the result of improvements in the performance of the total system. For example, specimen scanning rasters of 1000 line and 2500 lines are common, with additionally selectable, variable interlace patterns, either sequential or non-sequential, depending upon the particular advantages sought from the system. (See, for example, U.S. Pat. Nos. 3,767,926 (Re.28,153) and application Ser. No. 534,975 filed 12/20/74.
It has also been determined that a variety of additional information may be gained from a specimen if, while scanning the specimen for response in a particular response (e.g. angle of beam impingent, secondly, backscattered electron detector, x-ray mapping) and direct comparison with a collateral specimen response (such as above, or scanning transmission bombardment) additional information might be gained from the specimen. The ability of the field emission microprobe system to respond with real time response lends additional advantages to the above investigations -- particularly for simultaneous comparative viewing such as in a stereoscoptive type of presentation.
The present invention is an improvement of an electron optical system adopted to a TV type imaging device and synchronized in raster pattern to the system electron probe.
The present system finds particular advantage in such systems as those in which two immediate images need to be compared such that simultaneous viewing gives the viewer information beyond that available from single images. Particularly, in those situations where separate images of an object are formed such that when these are viewed in particular relation, stereopsis may be observed, the viewer gains depth information which cannot be obtained from the single image. The current development of the electron microscope art is such that separate, coordinated photomicrographs may be prepared and later compared in suitable viewing device such as the stereoptic information can be extracted. Considerable research efforts have been expended in recent years toward the development of a real time stereo capability in a high magnification electron microscope. Real time systems capable of operation in lower magnifications (2000.times.) have been achieved with multicolor systems or with comparatively low resolution.
It is not until the present advance that a high resolution, high magnification scanning electron microscope has become available which could be developed as a satisfactory indistrial or research quality instrument.
In addition the present invention offers the flexibility of an electron microscope which allows the simultaneous viewing of an image which is formed from the different returns of two electron detectors. Specific examples may include the viewing of a backscattered electron image with a secondary electron image, in side-by-side relation and simultaneously.
It is thus one of the several objects of the present invention to provide such simultaneous viewing of two images of the same selected surface area of sample, however simultaneously producing separate images, carrying different information such that direct comparison may be made of the images with the assurance that the same specific surface of the specimen is being viewed.